Low power capacitive sensor button

ABSTRACT

Disclosed herein are system, methods, and apparatus for low power capacitive sensors. Apparatus may include a timing block configured to generate a repetitive trigger signal having a first frequency, and further configured to generate a clock signal having a second frequency. Apparatus may also include a sensing block coupled with the timing block and configured to, in response to the repetitive trigger signal, detect a change in capacitance associated with an object proximate to a capacitive sensor button by applying an excitation signal to the capacitive sensor button during a measurement period. Apparatus further include a wake logic block coupled with the sensing block and configured to transition a processing unit from a first power consumption state to a second power consumption state in response to the sensing block detecting the change in capacitance associated with the object proximate to the capacitive sensor button.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/667,434, filed on Mar. 24, 2015, which claims priority to U.S.Provisional Application No. 62/067,347, filed on Oct. 22, 2014, both ofwhich are incorporated by reference herein in their entirety.

TECHNICAL FIELD

This disclosure relates to the field of touch-sensors and, inparticular, to capacitive touch-sensing buttons.

BACKGROUND

Many electronic devices include a user interface device for allowinguser interaction and user input. One user interface device is a buttonor a key. Conventional buttons include mechanical components to actuatea switch to indicate a button press or button activation. Mechanicalbuttons also provide tactile feedback to the user to indicate the buttonhas been pressed. More recently, touch-sensor buttons are being used insome applications to replace mechanical buttons.

One type of touch-sensor button operates by way of capacitance sensing,utilizing capacitance sensor electrodes. The capacitance detected by acapacitance sensor changes as a function of the proximity of aconductive object on or near the sensor electrode. The conductive objectcan be, for example, a stylus or a user's finger. In a touch-sensorbutton, a change in capacitance detected by each sensor due to theproximity of a conductive object can be measured by a variety ofmethods. Typically, an electrical signal representative of thecapacitance detected by each capacitance sensor is processed by aprocessing device, which in turn produces electrical or optical signalsrepresentative of the button or sensor activation of the touch-sensorbutton.

However, mechanical buttons may still consume less power than capacitivesensor buttons because it is possible for mechanical buttons in theinactive state to not draw current, while still being responsive toinput. In contrast, monitoring touch presence for existing capacitivesensor buttons may depend on the operation of various analog and digitalcircuits, thus increasing power demand even when the buttons are notbeing touched.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings.

FIGS. 1A and 1B illustrate a capacitive sensor button, according to anembodiment.

FIGS. 1C and 1D illustrate a capacitive sensor button, according to anembodiment.

FIG. 2A is a block diagram illustrating an embodiment of a computingsystem that receives input via capacitive sensor buttons.

FIG. 2B is a timeline illustrating the operation of an embodiment of acomputing system that receives input via capacitive sensor buttons.

FIG. 3 illustrates an embodiment of a bridge circuit for detectingtouches at a capacitive sensor button.

FIG. 4A illustrates an embodiment of a resistor-capacitor (RC) circuitfor detecting touches at a capacitive sensor button.

FIGS. 4B and 4C illustrate voltage waveforms for a resistor-capacitor(RC) circuit for detecting touches at a capacitive sensor button,according to an embodiment.

FIG. 5A illustrates an embodiment of a capacitor divider circuit fordetecting touches at a capacitive sensor button.

FIGS. 5B and 5C illustrate voltage waveforms for a capacitor dividercircuit for detecting touches at a capacitive sensor button, accordingto an embodiment.

FIG. 6A illustrates an embodiment of a capacitor divider circuit fordetecting touches at a capacitive sensor button without a referencevoltage.

FIGS. 6B and 6C illustrate voltage waveforms for a capacitor dividercircuit for detecting touches at a capacitive sensor button without avoltage reference, according to an embodiment.

FIG. 7 is a flow diagram illustrating a process of detecting touches atone or more capacitive sensor buttons in a computing system, accordingto an embodiment.

DETAILED DESCRIPTION

The following description sets forth numerous specific details such asexamples of specific systems, components, methods, and so forth, inorder to provide a good understanding of several embodiments of thepresent invention. It will be apparent to one skilled in the art,however, that at least some embodiments of the present invention may bepracticed without these specific details. In other instances, well-knowncomponents or methods are not described in detail or are presented in asimple block diagram format in order to avoid unnecessarily obscuringthe present invention. Thus, the specific details set forth are merelyexemplary. Particular implementations may vary from these exemplarydetails and still be contemplated to be within the spirit and scope ofthe present invention.

In one embodiment of a computing system having one or more capacitivesensor buttons, a capacitance sensing module of the computing system mayoperate in a state where the computing system is responsive to input viathe one or more capacitive sensor buttons, while drawing an averagesteady state current that is no greater than 100 nanoamperes (nA). Inthis state, the capacitance sensing module monitors the one or morecapacitive sensor buttons to detect any conductive object in contactwith or in the proximity of any of the capacitive sensor buttons, andcan respond to a detected touch by waking a processing unit from a lowpower consumption state to a high power consumption state. Whenoperating in the high power consumption state, the processing unitconsumes more power than in the low power consumption state. In oneembodiment, the capacitance sensing module includes a low poweroscillator that draws a minimal amount of current for the majority ofthe button monitoring period and a sensing circuit that periodicallyuses a greater amount of current for only a short period of time. Thecapacitance sensing module may thus be optimized for minimizing currentconsumption rather than for signal-to-noise ratio and scan rate.

The low power oscillator in the capacitance sensing module operatesindependently from other clock resources in the computing system, thusallowing the other clock resources to be turned off to reduce powerdemand, even while the capacitive sensor buttons are being monitored. Inaddition, the capacitance sensing module contains sufficient circuitryand touch-detection logic for detecting button touches withoutassistance from a central processing unit (CPU) of the computing system,thus allowing the CPU to be halted and transitioned to a low powerconsumption state during the button monitoring period.

In one embodiment, such a capacitance sensing module having a reducedpower demand allows capacitive sensor buttons to be used in place ofmechanical switches with minimal impact to the power demand imposed onthe computing system. In particular, a low power capacitance sensingmodule may be used to implement capacitive sensor buttons in batterypowered devices or other devices where low power consumption is desired.

FIGS. 1A and 1B illustrate a first type of capacitive sensor button formeasuring self-capacitance that may be used with a low power capacitancesensing module, according to an embodiment. FIG. 1A illustrates a toplayer 101 and a bottom layer 102 of the self-capacitance sensor button,while FIG. 1B illustrates a profile section view 103 of the sensorbutton along axis 104. The self-capacitance sensor button as illustratedin FIGS. 1A and 1B includes the circular patterns of conductive materialin the top layer 101 and the bottom layer 102 which are attached to thetop and bottom surfaces of a substrate 105, respectively. The top layer101 includes a sensor electrode 106 that is substantially surrounded bya shield electrode 107 and that is electrically connected to aconnecting trace 109 that extends past the shield electrode 107 and canbe used for connecting the sensor electrode 106 to sensing circuitry inthe capacitance sensing module. The bottom layer 102 includes a shieldelectrode 108 having a cross-hatched fill pattern that overlaps the areacovered by electrode 106, electrode 107, and trace 109. The shieldelectrode 108 is electrically connected to the shield electrode 107, andthe shield electrodes 107 and 108 are grounded.

With reference to the profile view 103, the self-capacitance Cs 110represents a capacitance between the sensor electrode 106 and the shieldelectrodes 107 and 108. In one embodiment, the capacitance Cs 110 is inthe range from 3 picofarads (pF) to 5 pF, and changes by at least 1 pFwhen the button is touched by a conductive object such as a finger orstylus.

FIGS. 1C and 1D illustrate another type of capacitive sensor button formeasuring mutual capacitance between two electrodes that may be usedwith a low power capacitance sensing module, according to an embodiment.FIG. 1C illustrates a top layer 121 and a bottom layer 122 of themutual-capacitance sensor button, while FIG. 1D illustrates a profilesection view 123 of the sensor button along axis 124. Themutual-capacitance sensor button as illustrated in FIGS. 1C and 1Dincludes the patterns of conductive material in the top layer 121 andthe bottom layer 122 which are attached to the top and bottom surfacesof a substrate 125, respectively. The top layer 121 includes a receive(RX) sensor electrode 126 that is capacitively coupled with a transmit(TX) sensor electrode 127. The TX sensor electrode 127 and the RX sensorelectrode 126 are extended by traces 130 and 129, respectively, whichcan be used as connection points to connect the electrodes 127 and 126to sensing circuitry in the capacitance sensing module. The bottom layer122 includes a shield electrode 128 that is connected to ground and thatoverlaps the area covered by the RX electrode 126, TX electrode 127, andtrace 129 with a cross-hatched fill pattern.

With reference to the profile view 123, the mutual capacitance Cm 131represents a capacitance between the TX sensor electrode 127 and the RXsensor electrode 126, while the self-capacitance Cs 132 represents acapacitance between the RX sensor electrode 126 and the grounded shieldelectrode 128. In one embodiment, the mutual capacitance Cm 131decreases in response to a conductive object touching or in theproximity of the boundary between the TX electrode 127 and the RX sensorelectrode 126, while the self-capacitance Cs 132 increases.

In some embodiments, capacitive sensor buttons as described above may beoverlaid with a protective film, coating, or other material over the toplayer of conductive material of the sensor button. For example, a layerof plastic or glass may be used to protect the conductive material fromdirect contact. Thus, the sensor button may detect a conductive object,such as a finger or stylus, that contacts the surface of the overlaidmaterial rather than directly contacting the conductive material of thesensor button.

FIG. 2A illustrates a computing system 200, according to an embodiment.Computing system 200 includes capacitive sensor buttons 201, acapacitance sensing module 202, and a processing module 203. Thecapacitance sensing module 202 operates on a different power domain thanthe processing module 203, so that the capacitance sensing module 202may be operated in different power consumption states independently fromthe power consumption state of the processing module 203. Accordingly,the capacitance sensing module 202 may be operating during a buttonmonitoring period in order to monitor touches at the capacitive sensorbuttons 201 even while the processing module 203 and/or the remainder ofthe computing system 200 is maintained in a low power consumption state,such as a suspend, standby, or hibernate state.

In one embodiment, the capacitance sensing module 202 includes a timingblock 210 and a sensing block 220. The timing block 210 includes a lowpower oscillator 211 and a timer circuit 212 that run continuously torepeatedly trigger the sensing block 220 to determine whether a contactis present at any of the buttons 201 (i.e., determine whether aconductive object is contacting or in the proximity of one of thebuttons 201). FIG. 2B illustrates a timeline describing the operation ofthe timing block 210 and sensing block 220 during a button monitoringperiod 260, according to an embodiment, with time proceeding from leftto right. As illustrated in FIG. 2B, the duration of the buttonmonitoring period 260 includes repeated measurement periods 261 duringwhich the sensing block 220 is determining whether a button isactivated. The measurement periods 261 represent a relatively smallpercentage of the button monitoring period 260; thus, the operationalduty cycle of the sensing block 220 is relatively low, and the sensingblock 220 draws current for only a small percentage (e.g., 1%-10%) ofthe time during which the button 201 states are being monitored.

FIG. 2B also illustrates the power consumption 270 of the sensing block220 during the button monitoring period 260. The power consumption 270of the sensing block 220 is higher during the measurement period 261 andlower for the remainder of the button monitoring period 260 when nomeasurements are being performed. In one embodiment, a steady statecurrent drawn by the sensing block 220 during a measurement period 261is no more than 1 μA, and a steady state current drawn by the low poweroscillator block is no more than 10 nA during the button monitoringperiod 260. In alternative embodiments, the steady state current drawnby the low power oscillator block may be up to 12 nA or more. In oneembodiment, the total average steady state current drawn by the timercircuit, low power oscillator block, and the sensing block during thebutton monitoring period 260 is no more than 200 nA; in alternativeembodiments, this total average steady state current may be greater than200 nA. In one embodiment, this total average steady state current maybe less than 100 nA.

In one embodiment, the low power oscillator 211 draws no more thanapproximately 10 nanoamperes (nA) and generates an clock signal 213having a frequency that is no greater than 1 kilohertz (kHz). The lowpower oscillator 211 is connected to the timer circuit 212, and providesthe 1 kHz clock signal 213 to the timer circuit 212. In alternativeembodiments, the frequency of clock signal 213 may be greater or lessthan 1 kHz.

The timer circuit 212 receives the clock signal 213 from the low poweroscillator 211 and generates a repetitive trigger signal 214 based onthe clock signal 213. In one embodiment, the timer circuit may beimplemented by a clock divider or a counter to generate a repetitivetrigger signal 214 having a frequency that is less than the frequency ofthe clock signal 213. In one embodiment, the repetitive trigger signal214 may be a pulse train having a frequency not greater than 1 Hertz(Hz). In one embodiment, the repetitive trigger signal 214 is asubstantially periodic signal (i.e., having a fixed nominal period); inalternative embodiments, the repetitive trigger signal 214 may beaperiodic.

The timing block 210 transmits the clock signal 213 and the repetitivetrigger signal 214 to the sensing block 220. In response to therepetitive trigger signal 214, the sensing block 220 initiates ameasurement scan to determine whether a conductive object is in contactwith any of the capacitive sensor buttons 201. For example, for arepetitive trigger signal 214 that is implemented as a pulse train, thesensing block 220 may initiate a measurement scan in response to eachpulse in the pulse train, and may perform the measurement scan bysequentially applying one of the clock signals 213 or 225 to each of thecapacitive sensor buttons 201 to measure their respective capacitancevalues during the measurement period. As illustrated in FIG. 2B, thepulse 251 of the trigger signal 214 initiates the first of theillustrated measurement periods 261 during which the sensor block 220measures capacitance values from the capacitive sensor buttons 201.

The response of the sensing block 220 to the repetitive trigger signal214 is controlled by the state machine 221, which receives therepetitive trigger signal 214 from the timer circuit 212. For example,the state machine 221 may respond to a pulse of the repetitive triggersignal 214 by transitioning the sensing block 220 from a low powerconsumption state to a high power consumption state. In one embodiment,the low power consumption state is an operating mode of the sensingblock 220 in which the components of the sensing block 220, such as theoscillator 222, sensing circuitry 223, and wake logic 224 are notoperating and are drawing no current or minimal current. The statemachine 221 may thus transition the sensing block 220 to a high powerconsumption state by causing power to be supplied to the oscillator 222,sensing circuitry 223, and/or wake logic 224. By turning on thesecomponents of the sensing block 220, the state machine 221 causes theoscillator 222 to generate an clock signal 225 for the sensing circuitry223, and causes the sensing circuitry 223 to begin measurement of thecapacitive sensor buttons 201.

In one embodiment, the sensing circuitry 223 selects one of the clocksignals 214 or 225 and applies the selected clock signal to each of thecapacitive sensor buttons 201 in sequence to detect changes incapacitance resulting from a conductive object on or near any of thebuttons 201. In one embodiment, the low power oscillator 211 consumes 10nA of current to generate the clock signal 213 having a frequency of 1kHz, while the oscillator 222 consumes 1 microampere (μA) to generatethe clock signal 225 having a frequency of 100 kHz.

The use of clock signal 213 instead of clock signal 225 may result inrelatively lower power consumption and an increased measurement period,corresponding to a slower response time for detecting a button contact.In one embodiment where only the 1 kHz clock signal 213 is used, theoscillator 222 may also be omitted or maintained in the off state forall power consumption states to further reduce power consumption. Theuse of clock signal 225 instead of clock signal 213 may result inrelatively higher power consumption, a shorter measurement period, and aquicker response time for detecting a button contact.

In one embodiment, the state machine 221 is additionally configured totransition the sensing block 220 back to the low power consumption stateafter the measurement scan is complete and before a next subsequentpulse after the most recent pulse of the repetitive trigger signal 214.

The sensing block 220 includes a wake logic 224 which is configured tocause the processing unit 230 to transition from a low power consumptionstate to a high power consumption state in response to detecting thepresence of the conductive object at the one or more of the capacitivesensor buttons 201. The wake logic 224 transition the processing module203 from the low power consumption state to the higher power consumptionstate by outputting a wake signal to the processing unit 230. Forexample, wake logic 224 may transition the processing unit 230 from alow power consumption state that is an Advanced Configuration and PowerInterface (ACPI) C3 ‘sleep’ power state to a high power consumptionstate that is an ACPI C0 ‘operating’ power state.

In one embodiment, the processing unit 230 and/or other components ofthe processing module 203 are supplied power from a different powerdomain than the capacitance sensing module 202. By operating thecapacitance sensing module 202 and processing module 203 on differentpower domains, the modules 202 and 203 can be powered up and powereddown independently, and can operate in different power consumptionstates. In one embodiment, the processing module 203 is constructed on adifferent integrated circuit chip than the capacitance sensing module202. For example, a first integrated circuit chip that is supplied powerfrom a first power domain may include the timing block 210 and sensingblock 220, while a second integrated circuit chip that is supplied powerfrom a second power domain may include the processing unit 230 andmemory 231. In an alternative embodiment, the processing module 203 andthe capacitance sensing module 202 can be located on the same integratedcircuit chip.

In one embodiment, the wake logic 224 may determine whether a specificwake sequence has occurred, then output the wake signal to theprocessing module 203 in response to the wake sequence. A wake sequencemay be defined as, for example, activation of a particular button or acombination or sequence of buttons. If a valid wake sequence is notdetected, the wake logic 224 allows the processing unit 230 to remain inthe low power consumption state.

In one embodiment, the wake signal output from the wake logic 224 mayalso cause the processing module 203 to transition between low and highpower consumption states, since the processing module 203 may includeother components (such as memory 231) that can be switched between powerstates. For example, the wake signal may switch the processing module203 between one of the ACPI G1 ‘sleeping’ power states and the ACPI G0‘working’ power state. In one embodiment, the processing unit 230 maypropagate the wake signal to the other components of processing module203 in order to change the power consumption state of the processingmodule 203; alternatively, the wake signal may be received and processedby other logic in the processing module 203.

In one embodiment, the processing module 203 includes a memory 231storing instructions 232 that are executable by the processing unit 230.In one embodiment, the processing unit 230 is configured toautomatically execute the instructions 232 after transitioning from alow power consumption state to a high power consumption state. Theprocessing unit 230 may execute different sets of instructions dependingon the specific wake sequence that is detected by the wake logic 224.For example, when the sensing block 220 detects a contact at a firstcapacitive sensing button, the wake logic 224 may wake the processingunit 230 and cause the processing unit 230 to execute a first block ofinstructions, and when the sensing block 220 detects a contact at asecond capacitive sensing button, the wake logic 224 may wake theprocessing unit 230 and cause the processing unit 230 to execute adifferent block of instructions.

In one embodiment, the sensing circuitry 223 may be implemented using abridge circuit 300, as illustrated in FIG. 3. The bridge circuit 300includes a sensor branch 310 and a reference branch 320. Sensor branch310 includes impedances 311 and 312 and reference branch 320 includesimpedances 321 and 322. Node 313 between impedances 311 and 312 of thesensor branch 310 is connected to the positive input of comparator 302,and node 323 between impedances 321 and 322 of the reference branch isconnected to the negative input of comparator 302.

In one embodiment, the impedances 321 and 322 of the reference branch320 are programmable so that a ratio between the impedances 321 and 322differs from a ratio between the impedances 311 and 312 of the sensorbranch 310. For example, the impedances 321 and 322 may be programmed sothat the reference branch and sensor branch ratios differ by between 5%and 10%.

In one embodiment, the reference impedances 321 and 322 may beimplemented as programmable or adjustable impedances constructed on thesame integrated circuit chip as the sensing circuitry 223. For example,the reference branch impedances 321 and 322 may be implemented using aset of switchable capacitors that can be adjusted in a firmwareprocedure when the processing unit 230 is active. Alternatively, thereference impedance 322 may be an external structure having similarcharacteristics as the capacitive sensor button.

According to the operation of the bridge circuit 300, the excitationsignal 301 is applied to both the sensor branch 310 and the referencebranch 320. In one embodiment, the excitation signal 301 is analternating current (AC) signal that is generated by a digital logicunit based on one of the clock signals 213 or 225, and having the samefrequency as the clock signal 213 or 225 from which it was derived.During the operation of the bridge circuit 300, a conductive object incontact with the capacitive sensor button affects the timing response ofthe sensor branch 310 such that the voltage polarity across the inputsof the comparator 302 is reversed. The output of comparator 302 thusindicates whether the capacitive sensor button has been touched.

In one embodiment, the bridge circuit 300 is implemented as aresistor-capacitor (RC) circuit as illustrated in FIG. 4A. The RCcircuit 400 as illustrated in FIG. 4A may be used to detect changes inthe self-capacitance of a capacitive sensor button due to the proximityor touch of a conductive object. With reference to FIG. 1B, for example,node 413 of circuit 400 can be electrically coupled with the sensorelectrode 106 in order to detect changes in Cs 110.

The RC circuit 400 includes a reference branch 420, including aresistance 421 connected by a node 423 to a reference capacitance 422.These correspond respectively to the reference branch 320, impedance321, node 323, and impedance 322 in the bridge circuit 300. The RCcircuit 400 also includes a sensor branch 410, including a resistance411 connected by node 413 to the sensor capacitance 110, whichcorrespond respectively to the sensor branch 310, impedance 311, node313, and impedance 312 of the bridge circuit 300.

According to the operation of the RC circuit 400, the excitation signalsource 401 generates an excitation signal VTx, which is applied to boththe reference branch 420 and the sensor branch 410. The voltages VR andVA from nodes 423 and 413 are connected to the positive and negativeinputs of the comparator 402, respectively. The time constant of thesensor branch 410 changes in response to the presence of a conductiveobject contacting or in the proximity of the capacitive sensor button,and the voltage polarity at the inputs of comparator 402 is reversed,relative to when the button is not being touched.

The XNOR gate 403 asserts its output when the comparator 402 output Voutand the excitation signal VTx are both asserted or both deasserted. Theoutput of the XNOR gate 403 is then sampled by the Strob signal via ANDgate 404. The output Dout of AND gate 404 is a train of positive pulseswhen a touch is detected, and is a constant low voltage when no touch isdetected. FIG. 4B illustrates the voltages VTx, VA, VR, Strob, and Doutof RC circuit 400 when no touch is detected at the capacitive sensorbutton, and FIG. 4C illustrates the same voltages when a touch isdetected.

In one embodiment, the bridge circuit 300 is implemented as a capacitordivider circuit as illustrated in FIG. 5A. The capacitor divider circuit500 includes a reference branch 520, including capacitances 521 and 522connected together at node 523. These correspond respectively to thereference branch 320, impedance 321, impedance 322, and node 323 in thebridge circuit 300. The capacitor divider circuit 500 also includes asensor branch 510, including capacitances 131 and 132 connected togetherat node 513, which correspond respectively to the sensor branch 310,impedance 311, impedance 312, and node 313 of the bridge circuit 300.

The capacitor divider circuit 500 as illustrated in FIG. 5A may be usedto detect changes in the mutual capacitance and self-capacitance of acapacitive sensor button due to the proximity or touch of a conductiveobject. With reference to FIGS. 5A and 1D, for example, the excitationsignal generator 501 can apply the excitation signal VTx to the TXsensor electrode 127 while node 513 of circuit 500 is electricallycoupled with the RX sensor electrode 126 in order to detect changes inCm 131 and Cs 132.

Resistors 524 and 514 connect nodes 523 and 513, respectively, to areference voltage Vref to initialize and maintain the voltages VR and VAwithin the operating range of the comparator 502. The voltages VR and VAare applied to the positive and negative inputs of comparator 502. Whilethe excitation signal source 501 applies the excitation signal VTx tothe sensor branch 510 and the reference branch 520, a conductive objectcontacting the capacitive sensor button decreases Cm 131 and increasesCs 132. As a result, the voltage polarity at the inputs of comparator502 is reversed, relative to when the button is not being contacted. Thecomparator 502 output voltage Vout thus indicates whether or not aconductive object is touching or in proximity to the capacitive sensorbutton. A Vout signal indicating a button touch may be converted to apulse train using an XNOR gate and an AND gate, similar to the XNOR gate403 and AND gate 404 in RC circuit 400.

FIG. 5B illustrates the voltages VTx, VA, VR, Strob, and Dout of thecapacitor divider circuit 500 when no touch is detected at thecapacitive sensor button, and FIG. 5C illustrates the same voltages whena touch is detected.

FIG. 6A illustrates an embodiment of a capacitor divider circuit 600.Similar to circuit 500, the capacitor divider circuit 600 operates byapplying an excitation signal VTx to a reference branch 620 and a sensorbranch 610, then comparing the voltages VR and VA at nodes 623 and 613,respectively, using a comparator 602. However, the capacitor dividercircuit 600 consumes less power than circuit 500 by eliminating thereference voltage Vref and resistors 524 and 514.

Without the resistors 524 and 514, the capacitor divider circuit 600initializes and maintains the voltages VR and VA within the operatingrange of the comparator 602 using switches 624 and 614. The switches 624and 614 are closed during an initialization stage of the detectionprocess and are open during a measurement stage of the detectionprocess. During the initialization stage, nodes 623 and 613 areconnected together to eliminate any voltage difference between VR andVA.

The switches 624 and 614 are operated by a control signal C, which isgenerated from a gate 632 performing an XOR operation on the excitationsignal and a delayed version (via delay 631) of the excitation signal.This results in a signal C that resembles an inverted pulse train, withthe falling edge of each pulse substantially coinciding with the risingedge of the excitation signal VTx, and with the pulse width determinedby the duration of the delay 631.

FIGS. 6B and 6C illustrate voltage waveforms for the circuit 600 when atouch is not present and when a touch is present at the capacitivesensor button, respectively. As illustrated in FIG. 6B, theinitialization stage 652 corresponds to the time when the switches 624and 614 are closed by the control signal C and the difference between VRand VA is reduced to 0V. The measurement stage 651 corresponds to thetime when the switches 624 and 614 are opened by the control signal C.

During the measurement stage 651, the sign of the difference between VRand VA is determined by the whether a conductive object is in contactwith or in the proximity of the capacitive sensor button. Thus, thecomparator 602 output Vout indicates whether the capacitive sensorbutton has been contacted. A Vout signal indicating a button touch maybe converted to a pulse train using an XNOR gate and an AND gate,similar to the XNOR gate 403 and AND gate 404 in RC circuit 400.

FIG. 7 is a flow diagram illustrating an embodiment of a button sensingprocess 700. Process 700 may be performed by the computing system 200,including the components of the capacitance sensing module 202 and theprocessing module 203.

Process 700 begins at block 701. At block 701, the low power oscillator211 generates the clock signal 213 and supplies the clock signal 213 tothe timer circuit 212 and the sensing block 220. From block 701, theprocess 700 continues at block 703.

At block 703, the timer circuit 212 receives the clock signal 213 andgenerates the repetitive trigger signal 214 having a lower frequencythan the clock signal 213. In one embodiment, the repetitive triggersignal 214 is a pulse train. The repetitive trigger signal 204 isprovided to the state machine 221 in sensing block 220. From block 703,the process 700 continues at block 705.

At block 705, the state machine monitors the repetitive trigger signal204 for at least the duration of the button monitoring period that doesnot coincide with a measurement period. The operations of blocks 701 and703 are also continued for the duration of the button monitoring period.The process 700 does not continue to the next block 707 until the statemachine 221 detects a trigger pulse in the repetitive trigger signal214.

In response to the trigger pulse of the repetitive trigger signal 214,the sensing block detects the presence of a conductive object at acapacitive sensor button (e.g., one of the capacitive sensor buttons201) by applying an excitation signal VTx to the capacitive sensorbutton. Thus, the detection of the trigger pulse by the state machine221 initiates a measurement period.

The measurement period begins at block 707, when the state machine 221transitions the sensing block 220 from a low power consumption state toa high power consumption state. In one embodiment, the state machine 221performs this transition by supplying power or otherwise enabling thecomponents of the sensing block 220, such as the sensing circuitry 223,and wake logic 224. In embodiments that include oscillator 222,transitioning to a high power state also includes enabling oscillator222, allowing the oscillator 222 to draw current and generate clocksignal 225. From block 707, the process 700 continues at block 709.

At block 709, the sensing circuitry 223 generates an excitation signalVTx based on clock signal 213 or, in embodiments including an oscillator222, clock signal 225. The excitation signal may be generated within thesensing circuitry 223 by an excitation signal source such as excitationsignal source 401, 501, or 601. In one embodiment, the generatedexcitation signal VTx has the same frequency as the clock signal 213 or225 from which it is generated. From block 709, the process 700continues at block 711.

At block 711, the excitation signal source applies the generatedexcitation signal VTx to a reference branch (such as reference branch420, 520, or 620) and to a capacitive sensor button via a sensor branch(such as sensor branch 410, 510, or 610) of the sensing circuit 223.From block 711, the process 700 continues at block 713.

At block 713, a comparator of the sensing circuit 223 compares a voltageVR from a node of the reference branch with a voltage VA from a node ofthe sensor branch. With reference to FIG. 4A, for example, thecomparator 402 compares the voltage VR from node 423 with the voltage VAfrom node 413. Similarly, comparators 502 and 602 compare the voltagesfrom reference branch nodes 523 and 623 with voltages from sensor branchnodes 510 and 610, respectively, in FIGS. 5A and 6A. The comparatoroutput thus indicates whether or not a conductive object is contactingor in close proximity to the capacitive sensor button. In oneembodiment, blocks 709, 711, and 713 may be repeated to sense multiplecapacitive sensor buttons. From block 713, the process 700 continues atblock 715.

At block 715, the wake logic 224 determines based on the sensing circuit223 output Vout or Dout whether or not the capacitive sensor button hasbeen contacted. In embodiments where more than one button is sensed, thewake logic 224 may determine whether a valid wake sequence has beenperformed, such as activation of a specific button or specific sequenceof buttons. At block 715, if no button contact or valid wake sequencehas been detected, the process 700 continues to block 717. In someembodiments, the process 700 may repeat blocks 709-715 for multipleiterations prior to continuing at block 717, in order to detect buttontouches over a longer measurement period.

At block 717, the state machine 221 transitions the sensing block 220from the high power consumption state to the low power consumptionstate. The state machine 221 may transition the sensing block 220 to thelow power consumption state by turning off (e.g., disconnecting powerfrom or otherwise disabling) the components of the sensing block 220,such as the sensing circuitry 223 and wake logic 224. In embodimentsthat include oscillator 222, the transition to the low power consumptionstate may also include turning off the oscillator 222, ceasing thegeneration of clock signal 225. In one embodiment, the duration of themeasurement period is less than the period between trigger pulses of therepetitive trigger signal 214. Accordingly, the transition of thesensing block 220 to the low power consumption state occurs after themeasurement period and before a next subsequent pulse of the repetitivetrigger signal after the pulse of the repetitive trigger signal thatinitiated the most recent measurement period.

If, at block 715, a button touch or valid wake sequence is detected, theprocess 700 continues at block 719. At block 719, the wake logic 224transmits a wake signal to the processing unit 230, causing theprocessing unit 230 to transition from a low power consumption state toa high power consumption state. In one embodiment, the processing unit230 is maintained in a low power consumption state, (e.g., an ACPI C3‘sleep’ power state) during the button monitoring period. In response toreceiving the wake signal from the wake logic 224, the processing unit230 transitions to a high power consumption state (e.g., an ACPI C0‘operating’ power state). In one embodiment, the wake logic 224 alsocauses other components in the processing module 203 to transition froma low power consumption state to a high power consumption state inresponse to a button touch or wake sequence. From block 719, the process700 continues at block 721.

At block 721, the processing unit 230 operates normally in the highpower consumption state, and may execute instructions 232 retrieved frommemory 231. In one embodiment, instructions 232 may define one or moreapplications that operate based on input received from the capacitivesensor buttons. From block 721, the process 700 continues at block 723.

At block 723, the wake logic 224 determines whether the processing unit230 has transitioned back to the low power consumption state. If theprocessing unit 230 is not in the low power consumption state, theprocess 700 continues back to block 721. The processing unit 230 thuscontinues to function in the normal operating mode until it istransitioned to the low power consumption state by a user, time delay,or some other system process. If, at block 723, the processing unit 230is in the low power consumption state, the process 700 continues toblock 717.

At block 717, the state machine 221 transitions the sensing block 220from a high power consumption state to a low power consumption state. Asillustrated in FIG. 7, the sensing block 220 thus remains in a highpower consumption state in order to continue monitoring for contacts atthe capacitive sensor buttons while the processing unit 230 is operatingnormally in the high power consumption state according to block 721.After the processing unit 230 is transitioned back to the low powerconsumption state, the button monitoring period begins and the sensingblock 220 also transitions to a low power consumption state. The sensingblock 220 remains in this low power consumption state until the nextmeasurement period is triggered by the next trigger pulse of therepetitive trigger signal 214.

In an alternative embodiment, the sensing block 220 may be kept in a lowpower consumption state during normal high power consumption operationof the processing unit 230. According to such an embodiment, the sensingblock 220 can be transitioned to the low power consumption state after abutton touch or valid wake sequence is detected at block 715.

In the foregoing embodiments, various modifications can be made; forexample, TX sensor electrodes and RX sensor electrodes may beinterchanged, and physical features of the electrodes may be alteredrelative to the illustrated embodiments. As described herein, a“contact” or “touch” of a capacitive sensor may refer to any contact orpresence of an object that is sufficiently near the sensor to have aneffect on the capacitance that is measurable by the describedcapacitance sensing devices or circuitry. As described herein,conductive electrodes that are “electrically connected” or “electricallycoupled” may be coupled such that a relatively low resistance conductivepath exists between the conductive electrodes. The terms “substantially”and “approximately” may indicate values or characteristics that maydeviate from a nominal value or ideal characteristic (where suchdeviation may result from manufacturing tolerances, rounding error, andthe like) while the desired effect of the nominal value or idealcharacteristic is preserved.

Embodiments of the present invention, described herein, include variousoperations. These operations may be performed by hardware components,software, firmware, or a combination thereof. As used herein, the term“coupled to” may mean coupled directly or indirectly through one or moreintervening components. Any of the signals provided over various busesdescribed herein may be time multiplexed with other signals and providedover one or more common buses. Additionally, the interconnection betweencircuit components or blocks may be shown as buses or as single signallines. Each of the buses may alternatively be one or more single signallines and each of the single signal lines may alternatively be buses.

Certain embodiments may be implemented as a computer program productthat may include instructions stored on a computer-readable medium.These instructions may be used to program a general-purpose orspecial-purpose processor to perform the described operations. Acomputer-readable medium includes any mechanism for storing ortransmitting information in a form (e.g., software, processingapplication) readable by a machine (e.g., a computer). Thecomputer-readable storage medium may include, but is not limited to,magnetic storage medium (e.g., floppy diskette); optical storage medium(e.g., CD-ROM); magneto-optical storage medium; read-only memory (ROM);random-access memory (RAM); erasable programmable memory (e.g., EPROMand EEPROM); flash memory, or another type of medium suitable forstoring electronic instructions.

Additionally, some embodiments may be practiced in distributed computingenvironments where the computer-readable medium is stored on and/orexecuted by more than one computer system. In addition, the informationtransferred between computer systems may either be pulled or pushedacross the transmission medium connecting the computer systems.

Although the operations of the method(s) herein are shown and describedin a particular order, the order of the operations of each method may bealtered so that certain operations may be performed in an inverse orderor so that certain operation may be performed, at least in part,concurrently with other operations. In another embodiment, instructionsor sub-operations of distinct operations may be in an intermittentand/or alternating manner.

In the foregoing specification, the invention has been described withreference to specific exemplary embodiments thereof. It will, however,be evident that various modifications and changes may be made theretowithout departing from the broader spirit and scope of the invention asset forth in the appended claims. The specification and drawings are,accordingly, to be regarded in an illustrative sense rather than arestrictive sense.

What is claimed is:
 1. An apparatus comprising: a timer circuitconfigured to generate a repetitive trigger signal having a firstfrequency; an oscillator configured to generate a clock signal having asecond frequency; a sensing circuitry block coupled with the timercircuit and the oscillator and configured to, in response to therepetitive trigger signal, detect a change in capacitance associatedwith an object proximate to a capacitive sensor button by applying anexcitation signal to the capacitive sensor button during a measurementperiod; and a wake logic coupled with the sensing circuitry block, thewake logic configured to transition a processing unit from a first powerconsumption state to a second power consumption state in response to thesensing circuitry block detecting the change in capacitance associatedwith the object proximate to the capacitive sensor button.
 2. Theapparatus of claim 1, wherein an average current drawn by the sensingcircuitry block during the measurement period no more than 1 uA.
 3. Theapparatus of claim 1, wherein the oscillator comprises a low poweroscillator block configured to generate the clock signal.
 4. Theapparatus of claim 3, wherein an average current drawn by the low poweroscillator block is no more than 10 nA, wherein a combined current drawnby the timer circuit, the low power oscillator block, and the sensingcircuitry block is less than 200 nA.
 5. The apparatus of claim 3,wherein the low power oscillator block is further configured to providethe clock signal to the timer circuit, and wherein the timer circuit isfurther configured to generate the repetitive trigger signal based onthe clock signal.
 6. The apparatus of claim 1, wherein the first powerconsumption state is a first Advanced Configuration and Power Interface(ACPI) power state, and wherein the second power consumption state is asecond ACPI power state.
 7. The apparatus of claim 1, wherein the secondpower consumption state is configured to consume less power during aperiod of time than the first power consumption state.
 8. The apparatusof claim 1, wherein the sensing circuitry block further comprises: astate machine configured to transition the sensing circuitry block fromthe first power consumption state to the second power consumption statein response to a pulse of the repetitive trigger signal; and a sensingcircuit coupled with the state machine, wherein, during operation of thesensing circuitry block in the second power consumption state, thesensing circuit is configured to apply the excitation signal to thecapacitive sensor button during the measurement period.
 9. The apparatusof claim 8, wherein the state machine is configured to transition thesensing circuitry block to the first power consumption state after themeasurement period and before a next subsequent pulse of the repetitivetrigger signal.
 10. The apparatus of claim 1, wherein a sensing circuitof the sensing circuitry block comprises: a reference branch; a sensorbranch; and a comparator coupled with the reference branch and thesensor branch, wherein the sensing circuit is configured to: apply theexcitation signal to the reference branch, apply the excitation signalto the capacitive sensor button via the sensor branch, and compare avoltage at a first node in the reference branch with a voltage at asecond node in the sensor branch.
 11. The apparatus of claim 1, whereinthe sensing circuitry block is configured to cause a processing unit totransition from the first power consumption state to the second powerconsumption state in response to detecting a change in capacitanceassociated with an object proximate to the capacitive sensor button. 12.A method comprising: generating a repetitive trigger signal; generatingan excitation signal having a higher frequency than the repetitivetrigger signal; detecting a change in capacitance associated with anobject proximate to a capacitive sensor by applying the excitationsignal to the capacitive sensor during a measurement period, themeasurement period being triggered by a pulse of the repetitive triggersignal; and in response to detecting the change in capacitanceassociated with the object proximate to the capacitive sensor,transitioning a processing unit from a first power consumption state toa second power consumption state, the second power consumption statebeing a higher power consumption state than the first power consumptionsstate.
 13. The method of claim 12, further comprising: applying theexcitation signal to a reference branch; applying the excitation signalto the capacitive sensor via a sensor branch; and comparing a voltage ata first node in the reference branch with a voltage at a second node inthe sensor branch.
 14. The method of claim 12, wherein the first powerconsumption state is a first Advanced Configuration and Power Interface(ACPI) power state, and wherein the second power consumption state is asecond ACPI power state.
 15. The method of claim 12, further comprising:transitioning from the first power consumption state to the second powerconsumption state in response to a start of a first pulse of therepetitive trigger signal; and transitioning from the second powerconsumption state to the first power consumption state after themeasurement period and before a subsequent pulse of the repetitivetrigger signal after the first pulse of the repetitive trigger signal.16. A system comprising: a processing unit; a capacitive sensor; a timercircuit configured to generate a repetitive trigger signal having afirst frequency; a low power oscillator block configured to generate aclock signal having a second frequency; a sensing circuitry blockcoupled with the timer circuit and the oscillator block and configuredto initiate a measurement period in response to the repetitive triggersignal, and further configured to detect a presence of an object at thecapacitive sensor by applying an excitation signal, based on the clocksignal to the capacitive sensor, during the measurement period; and awake logic coupled with the sensing circuitry block, the wake logicconfigured to transition the processing unit from a first powerconsumption state to a second power consumption state in response to thesensing circuitry block detecting the presence of the object at thecapacitive sensor.
 17. The system of claim 16, wherein the timer circuitis configured to generate the repetitive trigger signal based on theclock signal, and wherein the repetitive trigger signal has a frequencynot greater than 1 Hz, and wherein the clock signal has a frequency notgreater than 1 kHz.
 18. The system of claim 16, further comprising amemory coupled with the processing unit, wherein the memory isconfigured to store instructions executable by the processing unit, andwherein the processing unit is configured to automatically execute theinstructions after transitioning from the first power consumption stateto the second power consumption state in response to the sensingcircuitry block detecting the presence of the object at the capacitivesensor.
 19. The system of claim 16, wherein the timer circuit, the lowpower oscillator block, the sensing circuitry block, and the wake logicoperate on a first power domain, and wherein the processing unitoperates on a second power domain independent from the first powerdomain.
 20. The system of claim 16, wherein the capacitive sensorfurther comprises: a transmit (TX) electrode coupled with the sensingcircuitry block; and a receive (RX) electrode coupled with the sensingcircuitry block and configured to capacitively couple with the TXelectrode based on the excitations signal, wherein a mutual capacitancebetween the TX electrode and the RX based on the excitation signaldecreases in response to a conductive object contacting a surface overthe TX electrode and the RX electrode.